1. Field of the Invention
This invention relates to theft detection apparatus of the type in which a protected object is detected by monitoring the magnetic fields produced by responder targets on protected articles when such articles are carried through an interrogation zone in which an alternating magnetic field is generated. In particular the invention comprises improvements relating to the responder targets and their manufacture.
2. Description of the Prior Art
French Pat. No. 763,681 to Pierre Arthur Picard discloses an article apparatus of the type to which this invention applies. As described in that patent, articles to be protected from theft are provided with responder targets in the form of thin strips of material having a high magnetic permeability and which are rapidly and repeatedly driven into and out of magnetic saturation in the presence of an alternating magnetic interrogation field. An interrogation antenna is provided at an interrogation zone in a passageway leading to the exit of a store or a protected area in a store; and means are provided to cause the interrogation antenna to generate an alternating magnetic field at a given frequency and at an intensity sufficient to saturate a responder target in the interrogation zone. As a result, the responder target itself produces alternating magnetic fields at frequencies which are harmonics of the given transmitter frequency. A receiver antenna is also provided at the interrogation zone to receive the magnetic fields produced by the responder target. The receiver antenna is connected to a receiver which is tuned to detect signals at one or more of the harmonic frequencies produced by the responder target; and an alarm is connected to the receiver to be activated when such detection takes place.
Various refinements to the basic device shown in French Pat. No. 763,681 are shown and described in U.S. Pat. Nos. 4,623,877, 4,074,249, 4,118,693, 3,820,103, 3,820,104, 3673,437, 3,737,735, 3,534,243 and 4,326,198.
The French Pat. No. 763,681 discloses that if the responder target is of elongated configuration, it will be saturated earlier, i.e. at a lower intensity of the interrogating magnetic field, than a short target or responder. U.S. Pat. No. 3,790,945 states that in the case of a responder target with dimensions providing a very high ratio of length to square root of cross-sectional area, the signal resulting from the presence of the very high harmonics is far greater than that resulting from greater amounts of the same materials having non-preferred dimensions. According to this last mentioned patent, the responder target (i.e., "marker") should have a high magnetic permeability (Permalloy metal is suggested); and it should be provided with a very slender cross-section as compared with length, as for example a cross-sectional area of 0.0004 square centimeters, and a length of 4 centimeters or more, this same being comprised in a ribbon not thicker than 0.00125 centimeters. This patent also states that the responder target is preferably provided with a ratio of length to square root of cross-sectional area which exceeds 200.
U.S. Pat. No. 4,568,921 discloses an improvement over the above described Permalloy targets. According to that patent, responder targets are provided which comprise a length of drawn wire of an easily saturable magnetic material which has been heat treated, after drawing, to substantially increase its magnetic permeability. This provides a target having an extremely small cross section which makes it economical to manufacture in that it does not require much material. Furthermore such a small cross section makes it easy to conceal the responder target on an item to be protected. Moreover the novel responder target of that invention produces a very distinct magnetic field disturbance which is easily detectable. Nevertheless it is always advantageous to provide targets which have increased detectability and reduced sensitivity to mechanical handling.
It has also been proposed to flatten cold drawn Permalloy wire so as to increase its squareness ratio and its coercive force so that the material can be used in a magnetic memory. This is described in "Metallurgical Control of Magnetic Properties in Co-Fe and Ni-Fe Alloy for Memory Applications" by G. Y. Chin et al., Journal of Applied Physics, Vol. 42, No. 4, pp. 1502-1509 Mar. 15, 1971. However, the resulting increase in coercivity obtained by this technique makes the material unusable for a theft detection sensor because the magnetic fields generated by a draft detection system would not be high enough to drive such high coercivity material into saturation.